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Geographic routing has proven to be an efficient and scalable routing protocol for wireless sensor networks due to exploiting pure location information of sensor nodes for routing data packets. To enable geographic routing, source nodes must be aware of the location of the sinks, which is typically achieved by a location service. In this paper, we propose an on-demand location service (OLS) for geographic routing in wireless sensor networks with mobile sinks. OLS divides the area covered by sensor nodes into virtual cells. Nodes can communicate with each other in a same cell. By using a lazy publish mechanism, sinks push their location information to relevant location server cells which are calculated dynamically. Source nodes determine the location of sinks from a declared location server if exists one in its neighbor cells. Otherwise, it will send a request packet to the home cell located in the area center. How to reduce the overhead and query response time in OLS is the challenge of this paper. Simulation results show that OLS not only provides an effective and efficient location service, but also consumes less bandwidth and shorter delay compared with HLS in scenarios with multiple mobile sinks and stationary source nodes.